The conventional bicycle crank assembly comprises a bottom bracket through which an axle extends, and two cranks are respectively connected to the two ends of the axle. A chainwheel is connected to the bottom bracket so that when the cranks are operated, the power is transferred to the wheel by the chainwheel, the chain and the gear on the rear wheel.
China Publication No. 101204984 discloses a bicycle crank assembly as shown in FIG. 11 and comprises a connecting tube “A” with an axle “C” extends therethrough, and two cranks “B” are respectively connected to the two ends of the axle “C”. The axle “C” is a hollow axle to reduce the weight. A protrusion “C1” is formed on the first end of the axle “C” and contacts the protrusion “B1” on the crank “B” in longitudinal direction to securely connect the crank “B” to the axle “C”. The longitudinal force is transferred to the protrusion “B1” via the protrusion “C1”, and then to the second end of the axle “C” and the other crank “B”. The force eventually is transferred to the bearing unit.
However, the longitudinal force is concentrated from the outside toward the inside of the combination, and the protrusion “C1” is the beginning point of the longitudinal force. The longitudinal force is reduced because tight fit between the parts of the combination. The concentration of the stress may break the axle “C”. FIG. 12 shows the cross sectional view of the crank and the axle. The application of the maximum stress to the two cranks and the axle is disclosed in FIG. 14. FIGS. 15, 16 and 17 are cross sectional views to show the application of the maximum stress to the two cranks and the axle. When a gap is formed between the protrusion “C1” and the protrusion “B1”, the longitudinal force cannot be performed as desired and the parts may slide, shift and loose. Besides, the protrusion “C1” is exposed and cannot prevent water, rain or corrosive liquid from entering the combination. Therefore, the existed connection between the cranks and the axle needs to be improved.